Electrical devices of all types are susceptible to damage from transient electrical events. The short duration of transient electrical events such as electrostatic discharge (ESD) might make them appear innocuous. However, this is simply not the case.
Take, for example, the high susceptibility of many integrated circuits to ESD events. ESD, as will be appreciated, is the rapid discharge of static electricity from one conductor to another conductor having a different electrical potential. The typical integrated circuit includes a variety of conductors and electrical components that are generally intended to operate near the same potential. Hence, components internal to the integrated circuit are not designed to withstand the rapid discharge of electricity arising during ESD events. However, when a lead of the integrated circuit comes into near contact with a conductor of a different potential (e.g., a human who has been walking over a carpet), an ESD event occurs potentially damaging the integrated circuit.
FIG. 1 provides a schematic useful for describing how an ESD event may arise in connection with an integrated circuit 10 of the prior art. The integrated circuit 10 includes primary circuitry 12, an ESD protection device 14, and an input lead 16 coupled electrically to both the primary circuitry 12 and the ESD protection device 14. The primary circuitry 12 and the ESD protection device 14 are both coupled to a common ground reference 20. An impedance Z1 models the effective impedance between the input lead 16 and an external conductor 30 that is the source of the static electricity.
When the external conductor 30 and the input lead 16 possess different potentials and are brought close enough together, the impedance Z1 is such that an ESD event occurs. The ESD event generates a current I.sub.ESD across the impedance Z1 and a voltage V.sub.ESD at the external conductor 30 with reference to the common ground 20. The typical ESD event lasts less than 300 nanoseconds, meaning that that the energy has dissipated and both the voltage V.sub.ESD and the current I.sub.ESD are negligible after this time. However, while the ESD event is occurring, the voltage V.sub.ESD and the corresponding I.sub.ESD can be exceedingly high. For example, the voltage V.sub.ESD from a normal human body discharge can exceed 3 kilo Volts.
Typical ESD protection devices such as ESD protection device 14 operate such that the resulting voltage V.sub.+ across the primary circuitry 12 is less than 100 Volts with the bulk of the current I.sub.ESD flowing through the ESD protection device 14 rather than the primary circuitry 12. For many integrated circuits, this reduction in applied voltage and the redirection of the current is sufficient protection to safeguard against any possible damage. However, certain integrated circuits are much more sensitive to transient electrical pulses and currently available ESD protection devices are not capable of providing the needed protection.
For example, certain integrated circuits include thin film fuses. These thin film fuses can be selectively blown, even after the integrated circuit has been packaged, in order to produce an integrated circuit having the desired electrical characteristics. This enables the electrical characteristics of the integrated circuit to be modified after the integrated circuit is fully packaged. As will be appreciated, different selections of the fuses result in different electrical characteristics. These thin film fuses are sensitive to voltage spikes and may be inadvertently blown when an ESD event occurs, even when protected by a standard ESD protection device. Once a fuse is improperly blown, the electrical characteristics of the integrated circuit are irreversibly altered.
Accordingly, what is needed is a filter circuit that can work in conjunction with common ESD protection devices to provide enhanced protection from ESD events and other transient electrical events.